Selective removal of dielectric materials and plating process using same

ABSTRACT

A metal is provided on a polymeric component and the component is subjected to a removal process such as plasma or liquid etching in the presence of an electric field. The etchant selectively attacks the polymer at the boundary between the metal and the polymer, thereby forming gaps alongside the metal. A cover metal may be plated onto the metal in the gaps. The cover metal protects the principal metal during subsequent etching procedures.

BACKGROUND OF THE INVENTION

The present invention relates to components for microelectronic devicesand to methods of making such components.

Many components used in microelectronic assemblies include fine leadswhich are used for connection to other elements of the device. Forexample, as taught in commonly assigned U.S. Pat. Nos. 5,148,265;5,148,266; 5,489,749; 5,536,909; 5,518,964 and 5,619,017 and PCTPublication WO/94/03036, the disclosures of which are herebyincorporated by reference herein, a microelectronic connection componentmay include a large number of electrically conductive terminals andleads disposed on a suitable support such as a dielectric sheet or acomposite element including both metal and dielectric layers. The leadsmay include connection sections projecting beyond edges of the supportor across apertures in the support. The connection sections may bebonded to contacts on a semiconductor chip to thereby connect the chipcontacts to the terminals on the component. Most often, the leads areformed principally from a metal such as copper or a copper-based alloy.As disclosed, for example, in commonly assigned U.S. Pat. No. 5,597,470,it is often desirable to provide a layer of a cover metal over some orall of the surfaces of the principal metal portion of the lead.Depending upon the particular application, the cover metal may provideenhanced properties such as easier bonding of the leads to chip contactsor other structures; enhanced fatigue resistance; or enhanced corrosionresistance.

One common procedure for making leads on a support utilizes a thinconductive layer, typically copper, on a dielectric layer such as adielectric layer of a rigid circuit panel or a flexible circuit panel,commonly referred to as a “tape”. A layer of photoresist is applied overthe conductive layer and patterned using conventional photographicprocesses to provide a series of openings in the form of elongated slotsat locations where the leads are to be formed. The slots in thephotoresist leave portions of the conductive layer at the bottom of eachslot exposed. The principal metal such as copper is then deposited inthe slots, typically by electroplating the principal metal onto portionsof the conductive layer exposed within each slot. The principal metaldeposited within each slot fills the bottom portion of the slot. A layerof the cover metal is deposited onto the top surface of the principalmetal deposit, facing away from the support by a further electroplatingstep. The resist is removed and the part is exposed to an etchant whichwill attack the conductive layer, thereby removing the conductive layerfrom regions between the leads. In a variant of this process, a layer ofthe cover metal is deposited on the conductive layer within each slotbefore deposition of the principal metal, so as to form a cover metallayer on the bottom surface of each principal metal deposit, facingtoward the support. After the etching step used to remove the conductivelayer, further cover metal may be deposited onto all of the leadsurfaces by a further electroplating step.

Typically, the etchant which is used to remove the conductive layer willnot attack the cover layer appreciably but will attack the principalmetal. The cover layer on the top surface of the principal metal willprotect the principal metal from the etchant to some degree. A coverlayer on the bottom surface can also provide some protection. However,the vertically-extending edge surfaces of the principal metal are notcovered by the cover metal, and these surfaces are attacked by theetchant. Loss of principal metal results in a lead having an irregularcross-sectional shape and “undercutting” or removal of principal metalfrom beneath the top cover layer, leaving portions of the top coverlayer projecting laterally at edges of the lead. Moreover, the principalmetal in the finished lead will have cross-sectional area smaller thanthe cross-sectional area of the original principal metal deposit. All ofthese phenomena tend to weaken the lead, and to reduce its electricalperformance. Moreover, the projecting portions of the top cover layercan break off of the lead, a phenomenon commonly referred to as“flaking”. This can cause short circuits between adjacent leads. Thesephenomena are subject to some variability depending due to variations inthe etching process. These phenomena and variations in these phenomenaare more significant in the case of fine leads, with small nominalcross-sectional dimensions.

Thus, there has been a need for a lead-forming process which willalleviate the problem of edge surface undercutting. Other metallicelements are also formed by processes similar to the lead-formingprocess discussed above. For example, metallic terminals are oftenformed on supports using a process which is the same as the conventionallead-forming process discussed above, except that the openings in thephotoresist layer may be in the form of circular discs, squares, ovalsor other desired terminal shapes rather than elongated slots. Theopenings used to form the terminals may be connected with the elongatedslot like openings used to form the leads, so as to form the terminalsintegral with the leads. Etching of the terminal edge surfaces presentsthe same problem as discussed above with reference to the leads. Similarproblems can occur in formation of still other conductive elements, andhence there has been a similar need for a fabrication procedure whichalleviates these problems.

Another procedure which is often used in fabrication of microelectroniccomponents is plasma etching of polymeric materials. For example, areactive plasma can be used to etch polyimide. A part having a polymericsurface is disposed within a plasma treatment chamber and a plasma isformed at subatmospheric pressure by an electrical discharge between apair of electrodes disposed within the chamber, or between an electrodeand a conductive wall of the chamber. Chemically reactive species formedwithin the plasma attack the polymeric material. This process can beemployed to form holes in a polymeric layer. As disclosed in copending,commonly assigned U.S. patent application Ser. No. 09/020,750, filedFeb. 9, 1998, the disclosure of which is hereby incorporated byreference herein, such a process can also be used to remove polymericmaterial from beneath a lead on a polymeric support, so as to make thelead detachable from the support. It would be desirable to increase thespeed and efficiency of such a process. Moreover, it would be desirableto provide such a process with selectivity, so that the process attacksthe polymeric layer preferentially in regions adjacent to metallicfeatures on the polymeric layer.

Yet another procedure used in manufacture of microelectronic componentsis electrochemical stripping. Polymeric materials such as acrylicpolymers found in certain photoresists can be removed from the surfaceof an underlying structure by exposing the surface to a bath of a liquidstripper which attacks the polymer. It would be desirable to provide forselective removal of the photoresist adjacent to metallic features.

SUMMARY OF THE INVENTION

One aspect of the invention provides a method of etching a polymericarticle having metallic features at a surface of the polymeric articlecomprising the steps of exposing the surface having the metallicfeatures to a plasma reactive with the polymer constituting the surface.Preferably, the plasma is maintained by applying electrical energy suchas an alternating electric field, typically at radio frequencies (“RF”)or other frequencies. The plasma selectively etches the polymer adjacentthe metallic features. In a method according to the this aspect of theinvention, the plasma may be an oxidizing plasma such as a plasmacontaining one or more constituents selected from the group consistingof oxygen and halogens, most preferably a plasma containing oxygen andfluorine. The polymer desirably is an organic polymer, and may be aphotoresist.

The method may include the step of applying an electrical potential tothe metallic features of the article being treated. The potentialapplied to the metallic features desirably is an alternating potential,such as an RF potential or lower frequency potential. The potentialapplied to the metallic features may provide some or all of the energyinput needed to sustain the plasma. Application of the potential to themetallic features enhances the selectivity of the etching process.

A further aspect of the invention provides a method of electrochemicallystripping a polymeric material from a surface of an article havingmetallic features as well as polymer at such surface. The methodaccording to this aspect of the invention desirably includes the stepsof contacting the surface of the article with a liquid etchant reactivewith the polymeric material in the article and applying an electricalpotential to metallic features of the article at the surface during thecontacting step so that the liquid etchant preferentially attacks thepolymer adjacent said metallic features. The etchant may be an ionic orpartially ionic liquid, such as an alkaline liquid of the type commonlyemployed in stripping photoresists. For example, the etchant may includeorganic tertiary amines such as triethanolamine. The polymer mayincludes polar moieties, and may be an acrylic polymer such as a resistof the type commonly used in electrophoretic deposition processes.

Although the present invention is not limited by any theory ofoperation, it is believed that the selectivity of the polymer removal inprocesses according to both of the foregoing aspects of the inventionarises at least in part from concentration of electrical charge orelectric fields at the juncture of the metal and polymer on the surface.Apparently, the discontinuity between the relatively high electricalconductivity of the metal and the far lower electrical conductivity ofthe polymer causes concentration of charge or fields at the juncturebetween the metal and the polymer. Regardless of the theory ofoperation, however, it has been found that the processes discussed abovepromote selective etching of the polymer adjacent the metal. Inparticular, where the metal feature at the surface has an edge surfaceextending downwardly into the polymer, the polymer is selectivelyremoved in an etch region extending downwardly into the polymeralongside the edge surface of the metal, so as to form gaps alongsidethe edge surfaces.

A further aspect of the invention provides methods of making metallicfeatures, such as leads or other metallic features of a microelectronicconnection component, on a support. A method according to this aspect ofthe invention desirably includes the steps of providing a resist on asurface of the support so as to leave openings in said resist atlocations where metallic features are to be formed, and depositing aprincipal metal on said support in the openings to form a principalmetal deposit with a top surface remote from the support and edgesurfaces extending downwardly from the top surface toward the support.The method further includes the step of treating the resist so as toremove resist adjacent the principal metal. For example, the surface ofthe article having the resist and principal metal may be subjected toselective removal treatments as discussed above.

Selective removal of the resist forms gaps between the edge surfaces andthe resist and thereby exposes the edge surfaces of the principal metaldeposit but leaves at least some of the resist on regions of the supportremote from the principal metal deposit. Following selective removal ofthe resist, a cover metal is deposited onto the exposed edge surfaces ofthe principal metal deposit. Preferably, the step of depositing thecover metal is performed so as to deposit cover metal on the top surfaceof the principal metal, facing away from the support as well as on theedge surfaces. Also, cover metal may be provided on the bottom surfaceof the principal metal by applying cover metal on the support prior toapplying the principal metal. Desirably, the cover metal on the top,bottom and edge surfaces of the support is continuous and completelysurrounds the principal metal. The method desirably further includes thesteps of removing the resist after the step of depositing the covermetal, typically by a nonselective process such as application of aconventional resist stripper or exposure to a plasma effective to stripthe resist.

The support may include a conductive layer and the step of depositingthe principal metal is performed so as to deposit the principal metalover the conductive layer. In this case the method may include the stepof etching the conductive layer to completely or partially release theprincipal metal deposit from the surface after the step of removing theresist. The cover metal protects the edge surfaces during the step ofetching the cover layer. Thus, the formed metallic feature has theintended shape, without the severe undercutting which has occurred incertain processes employed heretofore.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 through 5 are diagrammatic, fragmentary sectional viewsdepicting a component during successive stages of manufacture in aprocess according to one embodiment of the invention.

FIGS. 6 and 7 are fragmentary, diagrammatic perspective views depictingthe component of FIGS. 1-5.

FIGS. 8 through 10 are views similar to FIG. 1 but depicting a componentduring successive stages of manufacture in a process according toanother embodiment of the invention.

FIG. 11 is a diagrammatic, partially fragmentary view depicting acomponent during a stage of manufacture in a process according to yetanother embodiment of the invention.

FIG. 12 is a view similar to FIG. 11 but depicting a process inaccordance with yet another embodiment of the invention.

FIG. 13 is a fragmentary, diagrammatic view depicting a stage during aprocess according to a further embodiment of the invention.

FIGS. 14 and 15 are views similar to FIGS. 1-6 but depicting a processaccording to a further embodiment of the invention.

FIGS. 16 and 17 are views similar to FIGS. 14 and 15 but depicting aprocess according to a yet another embodiment of the invention.

FIGS. 18 and 19 are scanning electron microscope photographs depicting acomponent in accordance with an example of the invention during twosuccessive stages of its manufacture.

FIGS. 20 and 21 are scanning electron microscope photographs depicting acomponent during a process according to a further example of theinvention.

FIGS. 22 and 23 are scanning electron microscope photographs depicting acomponent during a failed process, shown as a comparative example.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A process according to one embodiment of the invention begins with abase or substrate 20 formed at least in part from a conventionaldielectric material such as a polymide, epoxy or other polymer.Substrate 20 has an electrically conductive layer 22 formed from areadily etchable metal such as copper or copper-based alloy on asurface. The substrate may include other conventional features such asinternal conductors, conductive potential reference planes and the like(not shown). A layer of a polymeric material such as a photoresist 24 isdeposited and subjected to a patternwise treatment such as aphotographic exposure and then cured in the conventional manner so as toleave openings 26 at locations where conductive features are to beformed.

For example, the photoresist 24 may be an electrophoretic photoresistsuch as those sold under the trademark Eagle 2100 ED Photoresist by theShipley Company of Marlborough, Mass. This photoresist can be depositedelectrophoretically onto conductive layer 22 by a conventionalelectrophoretic deposition process in which the component is immersed ina bath of an aqueous dispersion containing the resist. Conductive layer22 is maintained at positive electrical potential with respect to acounterelectrode (not shown) also immersed in the bath. Followingdeposition, the layer of photoresist is selectively patterned using aconventional photographic exposure process so that only selected areasof the deposited resist are cured. Following photographic exposure andpartial curing in this manner, the uncured resist is washed away,leaving openings 26 in the layer of resist 24. After stripping ofuncured resist, the resist may be further cured by baking at an elevatedtemperature, typically about 40-100° C. for a few minutes.

After the openings have been formed, a principal metal 28 is depositedwithin the openings by electroplating. Preferably, a bottom cover layer30 formed from a relatively etch-resistant cover material such as ametal selected from the group consisting of gold, osmium, rhodium,platinum, tin, nickel, chromium and alloys thereof is deposited withineach opening before the principal metal 28 is deposited. These metalsare deposited within the openings by immersing the component with theresist layer thereon into a conventional electroplating bath andapplying plating current through the conductive layer 22. The dimensionsof openings 26 define the dimensions of the deposited metal features inhorizontal directions parallel to the surface of conductive layer 22,whereas the amounts of metal deposited in the electroplating processesdefine the thicknesses of such features. These dimensions are selectedby the function to be performed by the metallic features in the finishedcomponent. For typical applications where the metallic features are tofunction as flexible leads, the metallic features typically are about15-60 microns wide and have a total thickness of about 10-40 microns.

As deposited the principal metal layer 28 has a top surface 32 facingaway from bottom cover layer 30 and conductive layer 22 and a pair ofedge surfaces 34 extending downwardly from the top surface 32. The edgesurfaces 34 closely conform to the surfaces of resist layer 24 definingthe opening 26. Stated another way, the component at this stage has apolymeric surface layer or resist 24 defining an exposed top surface 36and has a metallic feature including principal metal layer 28 exposed atthe top surface 36 and extending downwardly into the article from thetop surface.

In the next stage of the process, the article is immersed in a bath 38of a liquid etchant reactive with the polymer of layer 24 definingsurface 36. Where layer 24 is formed from a photoresist as discussedabove, the solution may be a solution of the type normally used toremove the photoresist. For example, a solution of the type sold underthe designation EPR Stripper 3460 by the Cherokee Chemical Company ofHawthorne, Calif. may be employed with a layer formed from theaforementioned Eagle 2100 photoresist. The EPR stripper includescomponents such as n-methy2-pyrrolidone, monoethanolamine and ethyleneglycol monobutyl ether. The stripper as supplied desirably is dilutedwith water typically about one part water to about four parts stripper.The bath desirably is maintained at about 70° C. (160° F.). In general,the concentration and temperature used with a given etchant should beslightly less than those used to provide rapid, nonselective attack bythe etchant on the polymer. A counterelectrode 40 is immersed in theetchant bath along with the component. Conductive layer 22 is maintainedat a negative electrical potential with respect to counterelectrode 40by a DC power source 42. Desirably, conductive layer 22 is maintained atabout 50 volts to about 200 volts with respect to counterelectrode 40.

Under these conditions, the etchant attacks the polymer of layer 24selectively adjacent to edge surfaces 34 of the principal metal layer28. Although the present invention is not limited by any theory ofoperation, it is believed that this selective attack is caused at leastin part by concentration of electric fields at the juncture between thehighly conductive metallic structure and the dielectric polymer, andconcentration of electrical charge at these locations. Regardless of thetheory of operation, continued exposure of the component to the etchantwith the electrical potential applied as discussed above forms gaps 44between the edge surfaces 34 of the principal metal 28 and the resist24. The width of these gaps is exaggerated in FIG. 2 for clarity ofillustration. In actual practice, the gaps form as relatively narrowslots adjacent the edges of the principal metal. As illustrated in FIG.2, the gaps may extend downwardly beyond the principal metal to theedges of the bottom cover material layer 30. The process desirably isterminated at about the time the gaps reach the conductive layer 22.That is, the gaps 44 should extend at least to the bottom of theprincipal metal layer 28. Where a bottom cover layer 30 is provided,gaps 44 should extend slightly beyond the bottom of the principal metallayer. The time required for gap formation will vary with the etchantcomposition; the polymer composition; and temperature and the voltageapplied, as well as on the thickness of the polymer layer and therequired depth of gaps 44. Typically, a few seconds to about a minute issufficient. The appropriate time can be established for any given systemby trials using various times. Once the process conditions have beenselected, the results typically are repeatable, so that the sameconditions can be used repeatedly in production operations.

After the resist has been selectively removed and the gaps have beenformed, the component is rinsed to remove the etchant and immersed in afurther electroplating bath and an additional layer of an etch-resistantcover material 46, such as a metal selected the group of cover metalsdiscussed above, is deposited on the edge surfaces 34 and top surface 32of the principal metal deposit 28. The additional cover metal 46 may bethe same as, or different from the cover metal of bottom cover metallayer 30. The additional cover metal 46 extends downwardly along theedge surfaces from top surface 32 and desirably extends all the way tothe bottom cover metal layer 30 so that the additional cover metal 36and the metal in the bottom cover layer 30 form a continuous jacket ofcover metal enclosing the principal metal 28. Here again, although thepresent invention is not limited by any theory of operation, it isbelieved that plating of the additional cover metal on the edge surfacesof the principal metal is promoted by concentration of electrical chargeand/or electric fields in gaps 44, at the junctures between the metallicstructures and the dielectric material in layer 24. Regardless of thetheory of operation, some of the cover material 46 is deposited on thelower portions of edge surfaces 34 despite the fact that these portionsof the edge surfaces are located deep within the relatively narrow gaps44. It is not essential that the additional cover material 46 have auniform thickness. However, the additional cover material desirablyforms a continuous layer over the entirety of the edge surfaces. Theplating process may be continued beyond the point where a continuouslayer is formed, so that the additional cover material entirely fillsgaps 44 and forms a relatively thick layer on the top surface 32 of theprincipal metal. Typically, the additional cover material is plated toabout 0.2 to about 5 microns thick on the top surface 32 of theprincipal metal.

After the additional cover metal has been applied, the polymericmaterial or resist 24 is entirely removed by any conventional processsuch as by prolonged exposure to a liquid etchant or resist stripper asdiscussed above, or by exposure to a reactive plasma. The conditionsused for stripping the photoresist layer in this stage may be moresevere than the exposure conditions used for the selective etchingprocedure discussed above with reference to FIG. 2. Thus, the strippermay be provided at a higher concentration and/or at a highertemperature, and the process may be continued for a longer time. Duringthis stage of the process, it is not essential to apply an electricalpotential to the conductive layer.

Once the resist has been entirely removed, the metallic features arepresent on the conductive layer 22 of the component. As seen in FIG. 4,each of the principal metal features 28 has a continuous jacket of covermaterial including the additional cover material 46 and the bottom coverlayer 30. The conductive layer 22 can then be removed from the polymericbody 20 by exposure to an etchant which is effective to attack theconductive layer but which does not substantially attack the protectivecover metal. For example, where conductive layer 22 is formed fromcopper, a conventional HCl/CuCl₂ etchant may be employed to remove theconductive layer. Removal of the conductive layer detaches the metallicelement from the dielectric body 20 as shown in FIG. 5. Thus, as shownin FIG. 5, the process yields a lead 50 overlying body 20.

As described in the aforementioned commonly assigned patents, such alead typically is not detached from the dielectric body 20 over itsentire length. Rather, the lead is formed in conjunction with otherfeatures, such as metallic vias extending into the polymeric body whichanchor the lead at one or both ends. Alternatively or additionally,portions of the conductive layer 22 may be left in place, as by maskingthese portions of the conductive layer during the step of removing theconductive layer, so as to leave ends of the leads anchored to thedielectric layer in the areas where the conductive layer is not removed.

Merely by way of illustration, the metallic features formed by theprocess as discussed above may include elongated leads 50 havingrelatively wide terminals 52 (FIG. 6) at a first end, having an anchorsection 54 at a second, opposite end and having connection sections 56narrower than the terminals extending from the terminals. The connectionsections are joined to the anchor sections by relatively narrowfrangible regions 58. All of these features of the metallic lead areformed by the process as discussed above with reference to FIGS. 1-4, sothat the metallic features overlie the metallic conductive layer 22 ofthe component. The leads are provided in an array so that the connectionsections 56 extend side by side. An aperture 48 is provided in thepolymeric body 20 in the form of an elongated slot extending beneath theconnection sections 56 and frangible sections 58 of the leads, as bylaser ablating or etching polymeric body 20. A masking layer 60 isprovided over anchor sections 54 of the leads and the adjacent areas ofconductive layer 22. After application of the masking layer and eitherbefore or after formation of slot 48, the component is immersed in theetching solution used to remove the conductive layer 22. The conductivelayer is etched entirely away in the areas overlying slot 48, and in theregions between terminals 52 and the adjacent sections of the leads, butthe conductive layer remains in place at the anchor sections. Thisleaves the leads in the condition illustrated in FIG. 7, with connectionsections 56 extending across gap 48 and with anchor regions 54 remainingattached to the polymeric layer 20 of the remaining portion ofconductive layer 22. During the etching process, portions of theconductive layer lying beneath the relatively wide terminals 52 areprotected from the etchant by the terminals and hence remain beneath theterminals, thus keep the terminals anchored to the polymeric body 20. Inother cases, the terminals may be formed integrally with metallic viasextending into or through the polymeric body. As described, for example,in the aforementioned U.S. Pat. Nos. 5,489,749 and 5,536,909, as well asin U.S. Pat. No. 5,619,017 and PCT Publication WO 94/03036, theindividual connection sections can be broken away from the associatedanchor regions by a bonding tool and bonded to contacts on asemiconductor chip. In the as-bonded condition, the individual leads areelectrically isolated from one another.

In other cases, the slotlike aperture discussed above may be replaced byindividual apertures associated with individual leads. In still othercases, the anchor sections are omitted. Alternatively, as described inU.S. Pat. No. 5,518,964, the leads may have terminal ends permanentlysecured to the dielectric component, as by vias extending into thedielectric component and may have tip ends detachably secured to thedielectric component so that the tip ends may be bonded to contacts on achip or other structure and the tip ends of all of the leads may bepulled away from the dielectric structure by moving the dielectricstructure with respect to the chip or other component. In a furthervariant, portions of the conductive layer 22 used to form the leads maybe left in place on the surface of substrate 20, such as in the regionsbetween the leads, so that these remaining portions of the conductivelayer act as a potential plane, such as a ground plane or power plane.These residual portions of the dielectric layer may be left in place byselectively masking the conductive layer prior to the etching step usedto remove the conductive layer. Thus, masking layer 60 (FIG. 6) mayinclude additional portions disposed between the terminal ends of theleads, and in other areas where the potential plane is to remain.However, the masking layer in these regions does not extend all the wayto the leads or terminals. Therefore, the conductive layer will beetched away in gaps surrounding the terminal ends of the leads, leavingthe resulting potential plane layer electrically isolated from theterminal ends of the leads.

A process according to a further embodiment of the invention uses acomponent incorporating a dielectric layer 120 and a conductive layer122 (FIG. 8). A resist or polymeric material 124 with openings 126 isprovided in substantially the same way as discussed above. Here again, abottom cover metal layer 130 and a principal metal 128 are deposited inopenings 126 so as to form metallic structures. At this stage of theprocess, the component includes a surface 136 defined by the polymericmaterial or resist 124 and a metallic structure exposed at such surfaceand having edges 134 extending downwardly from the exposed surfacetowards the conductive layer.

After the plating process, the resist may be subjected to a furtherbaking process, typically between about 40-100° C. and more typically atabout 50-80° C. for about 5-10 minutes to further cure the resist.Following such further curing, the resist desirably is exposed toultraviolet light at an intensity of about 10-200 mW/cm² for about 5sec-30 minutes. The applied ultraviolet light renders the resist moresusceptible to attack by the plasma, but does not completely degrade theresist to the point where the resist layer loses physical coherence.

After the metallic structures have been formed, the component issubjected to a plasma etching process in a chamber 137 (FIG. 9). Thechamber is provided with a conventional vacuum pump 139 and associatedconventional control system for maintaining a subatmospheric pressure. Agas source 141 is provided for supplying a gas mixture. Theplasma-forming gas supplied by source 141 desirably includes a halogencontaining gas such as SF₆ or, preferably a lower halogenatedhydrocarbon as, for example, tetrafluoromethane, and may also includeoxygen. optionally, an inert gas such as argon may be added. Forexample, the plasma-forming gas typically includes about 10-20% CF₄ andabout 70-80% O₂. The vacuum pump 139 and gas supply source desirably arearranged to move the etching gasses through the chamber at a ratesufficient to remove etched materials, and thereby limit redeposit ofetched materials onto the work.

The plasma chamber is also provided with a pair of electrodes 143 and145. One electrode is grounded, whereas the other electrode is isolatedfrom ground. An alternating potential source 147 is connected betweenthe electrodes. Typically, the chamber wall itself serves as a part orall of the ground electrode. In the particular arrangement illustratedin FIG. 9, the grounded electrode 143 includes a shelf supporting thecomponent. Potential source 147 is arranged to apply an alternatingpotential at a radio frequency formed desirably between about 40 KHz andabout 100 MHz. A so-called “ISM” frequency allocated by radiocommunications authorities for industrial process equipment, such as13.58 MHz, is commonly employed. The interior of the chamber desirablyis maintained at a submospheric pressure, typically a few milliTorr toone Torr. The applied electrical potential converts the gas to a plasmawhich includes highly reactive species such as monoatomic F and O, andions of such species. The plasma etches the polymeric material in layer144. Here again, the etching process is concentrated in regions adjacentthe junctures of the dielectric layer 124 with the conductive, metallicstructure 128. Thus, the polymeric material is etched at a greater rateadjacent the edge surfaces 134 of the principal metal 128, so that gaps144 are formed adjacent the edged surfaces. Some polymeric material isalso removed from regions of the layer 124 remote from the metallicfeatures, but the rate of such removal is markedly less than the rate ofremoval at the edge surfaces. Thus, gaps form adjacent the edge surfacesof the principal metal 128 and extend downwardly from the exposedsurface 136 of the polymer layer. Desirably, the process is continueduntil the gaps extend downwardly at least to the bottom cover metallayer 130. However, the exposure is terminated before the resist layer124 is completely removed in regions remote from the metallic structure128.

The exposure time required to form gaps extending into the polymer layerby the required distance depends upon the etching gases employed; uponthe composition of the polymeric layer; and plasma conditions such asapplied power, frequency pressure, flow rate, electrode structure andtime. Here again, the process is repeatable, so that once the process aconditions are established by trial and error, the same can be usedrepeatedly. Typically, about 5-20 minutes plasma etching, at an appliedpower of about 1,000 watts (0.05-1 watts/cm of exposed polymer surface)is sufficient to produce gaps of the required depth.

After this selective plasma etching process, additional cover metal 134is applied on the principal metal deposit 128 and covers the top surface132 as well as the edge surfaces 134 of the principal metal. Here again,the additional cover metal desirably merges with the bottom cover metallayer 130 to form a continuous jacket enclosing the principal metal 128.Once again, the remaining portions of the polymeric or photoresist layer124 mask those portions of the conductive layer 122 remote from thedeposited metal structure, so that the additional cover metal is notdeposited on these portions of the conductive layer. After theadditional cover material 146 is applied, the component may be furtherprocessed as discussed above.

In a process according to a further embodiment of the invention, theconductive layer 222 of the component (FIG. 11) is conductively coupledto one side of the alternating potential source 247, so that theconductive layer of the structure itself serves as one of the electrodesin the plasma excitation process. The opposite electrode 245 may beconnected to ground. For example, where the polymeric body of thearticle 220 is in the form of a thin sheet, the sheet may be held in arigid dielectric frame. As schematically shown in FIG. 3, the frame mayinclude a pair of ring-like elements 260 and 262. The dielectric sheet220 with the conductive layer 222 thereon is grasped between elements260 and 262. The dielectric frame holds the polymeric sheet taut andmaintains the sheet under tension. Frame element 262 has a hole 264extending through it. An electrode 268 connected to potential source 247is engaged with the conductive sheet 222 within hole 264. The use of aframe for processing flexible sheet-like elements is described ingreater detail in copending, commonly assigned U.S. Provisional PatentApplication No. 60/061,932 filed Oct. 17, 1997 and entitled Framed SheetProcessing, the disclosure of which is hereby incorporated by referenceherein.

Application of the alternating potential through the conductive layer222 serves to apply the potential to the metallic features extending tothe polymeric surface, such as the principal metal deposits 238extending to the polymeric surface 236. This in turn greatly accentuatesthe rate of etching at the junctures of the conductive feature and thesurrounding polymer as, for example, the rate of etching in gaps 244bordering the edge surfaces 234 of the metallic deposit 238. Desirably,the frequency of the alternating potential applied by potential source247 is selected so as to provide a good impedance match between thepotential source and the metallic features of the structure. Additionalinductance and/capacitance may be provided in conjunction with themetallic layer and other metallic features as, for example, by forminginductive or capacitive elements integrally with these components, or byconnecting additional conductive or capacitive components to the sheetand/or to other features. Additional electrical energy may be suppliedthrough additional electrodes in the plasma chamber, or through an RFcoil adjacent the plasma chamber, connected to the same alternatingpotential source or to another source operating at the same frequency ora different frequency. In a further variant, electrical energy appliedto the metallic structures may cause heating of the principal metaldeposit, as by inductive or resistive effects. Such heating selectivelyaccelerates etching of the polymer layer immediately adjacent themetallic structures.

As shown in FIG. 12, the alternating potential source may becapacitively coupled to the metallic features of the part being etchedrather than conductively coupled. Also, as shown in FIG. 13, thealternating potential source 247 may be inductively coupled to metallicfeatures of a part being etched. In the arrangement depicted in FIG. 13,the part being processed does not incorporate a continuous conductivelayer but instead incorporates individual metallic features conductivelyconnected to one another. Also, the particular metallic features are notelongated strips but are instead cylindrical via liners extendingthrough a dielectric layer formed from a polyimide or other structuraldielectric material rather than a dielectric resist. In this case, theselective etching process forms gaps in the resist around the vialiners. An inductive element 251 and a capacitive element 253 areconnected in circuit with these elements so as to form an L-C resonantcircuit. That circuit is inductively coupled to alternating potentialsource 247 through an inductive element 253 connected between the sourceand ground. The inductive coupling excites the resonant circuit and thussupplies varying potentials at each of the features 255. The potentialssupplied at the various features are not uniform in any given instant.The alternating potential source may also apply alternating potentialsto a counterelectrode 259 which is juxtaposed in the plasma chamber (notshown) with the component. Here again, the inductive and capacitivefeatures may be formed as parts of the object being treated, or else maybe separate components electrically connected to these features. Hereagain, the application of electrical potential to the metallic featuresaccelerates the etching at the interfaces between the metallic,conductive features and the surrounding dielectric material.

In a process according to a further embodiment of the invention, FIG.14, where the bottom cover layer is omitted, the principal metal 328 isdeposited directly onto the conductive layer 322, and the cover material346 plated onto the principal metal extends downwardly along the edgesurfaces 334 to the gaps 344 formed during selective removal of thepolymeric material. The subsequent etching process used to remove theconductive layer may be conducted so as to completely remove theconductive layer 322. Alternatively, as shown in FIG. 15, the etchingprocess may proceed only to the extent needed to remove the conductivelayer in areas remote from the deposited metallic structures. Theetching leaves a small portion 322′ of the conductive layer disposedbeneath the deposited principal metal lead or other structure. Hereagain, etch resistant cover metal 346 on the edge surfaces substantiallyprotects the principal metal from etching during this process. However,some of the principal metal is removed as those portions of theconductive layer 322 underlying the principal metal are etched away,thereby exposing the bottom surface of the principal metal to theetchant. A similar procedure, removing only a portion of the conductivelayer from beneath the deposited metal feature can be employed when thedeposited metal feature includes a bottom cover layer. Such a procedurecan be used, for example, to form a feature which is attached to theunderlying dielectric material 320 over only a relatively small area andhence is only weakly attached to the dielectric material. Features ofthis type are described in greater detail in certain embodiments ofcopending, commonly assigned U.S. patent applications Ser. No.09/195,371, filed Nov. 18, 1998 and Ser. No. 09/020,750 Feb. 9, 1998,the disclosures of which are hereby incorporated by reference herein.Other releasable lead structures using features attached over arelatively small area are disclosed in U.S. Pat. No. 5,518,964, thedisclosure of which is also incorporated by reference herein.

A method according to a further embodiment of the invention uses asubstrate having a polymeric layer 420 at a surface and metallicelements, such as elongated leads 428 seen in cross-section in FIG. 16,disposed on such surface. The selective removal of the polymer adjacentto the metal by a plasma or liquid etchant as described above forms gapsbeneath the leads. The process is terminated before the etchingprocedure completely detaches the leads from the polymeric layer. Thus,the process leaves each lead connected to the polymeric layer by a thinconnecting element 421 integral with the polymeric layer. As disclosedin the aforementioned copending, commonly assigned U.S. patentapplication Ser. No. 09/020,750, such a thin polymeric connectingelement can be used to provide a detachable lead structure. Thus, theleads can be bonded to a mating element such as a semiconductor chipwhile the leads are held in place on the substrate by connectingelements 421. After bonding, the substrate and the mating element can bemoved away from one another, thus peeling the leads away from thesubstrate and leaving the leads free to flex.

Numerous variations and combinations of the features discussed above canbe utilized without departing from the present invention as defined bythe claims. As these and other variations and combinations of thefeatures discussed above can be utilized without departing from thepresent invention as defined by the claims, the foregoing description ofthe preferred embodiments should be taken by way of illustration ratherthan by way of limitation of the invention as defined in the claims.

The following non-limiting examples further illustrates certain featuresof the invention.

EXAMPLE 1

A polyimide sheet with a 5-10 micron thick copper conductive layerthereon is coated with Eagle 2100 ED photoresist, and solvent is removedby baking. The coating is photographically exposed using 240-400milliJoules applied energy in a pattern including unexposed areas about10-100 microns wide. The resist is partially cured and then the uncuredresist in the unexposed areas is rinsed away leaving openingsapproximately 10-100 microns wide. Copper is plated to a thickness ofapproximately 16 microns within these openings. After plating, thecomponent is baked at 50° C. for 10 minutes and then exposed toultraviolet light at a wavelength of about 360-500 and at an intensityof approximately 30 mW/cm² for 5 minutes. The component is then exposedto plasma etching as discussed above with reference to FIG. 9 using aplasma forming gas containing approximately 30% CF₄; 70% O₂ at apressure of 250 milliTorr using 1,000 Watts applied power for 10minutes. The total exposed area of the polymeric surface being etchedwas approximately 400 cm².

The configuration of the component following plasma etching is shown inFIG. 18. The gaps extending downwardly along side of the metal depositsare indicated by arrows in the photographs. The component is then platedwith gold to a thickness of about 1-2 microns on the top surfaces of thedeposited copper. During this procedure, gold is also plated on the edgesurfaces of the deposited copper in the gaps. After gold plating, theresist is removed and the copper conductive layer is etched away byexposure to an HCl/CuCl₂ etchant for about 30-60 sec. In the resultingstructure, the copper conductive layer is entirely removed in regionsoutside of those covered by the leads. The copper conductive layer isalso partially removed beneath the lead, as shown in FIG. 19.

EXAMPLE 2

A component is processed as in Example 1 except that a bottom coverlayer of 2 micron thick gold is plated into the opening before thecopper is plated into the openings. Also, the resist is baked at 80° C.for 5 minutes and exposed to ultraviolet radiation at slightly more than100 mW/cm² for 10-15 seconds. The configuration after plating and plasmaetching is as shown in FIG. 20. Here again, the gaps formed along sidethe edge surfaces of the lead are indicated by arrows in thephotographs. Additional gold cover material is plated onto the structureafter formation of the gaps, to a thickness of about 0.5-2 microns onthe top surface of the copper. The additional cover material joins withthe bottom cover layer of gold and completely encases the copper orprincipal metal of the lead. The resist is then removed and theconductive layer is completely etched away by prolonged exposure toHCL/CuCl₂ solution. As shown in FIG. 21, the resulting lead is undamagedby the etching procedure used to remove the conductive layer. The copperprincipal metal is fully protected by the surrounding gold.

Comparative Example

A component is processed as in Example 2 except that the UV exposure is30 mW/cm² for 5 minutes. As seen in FIG. 22, the gaps formed by plasmaetching are narrower than those formed in Example 2 (FIG. 20) and do notextend all the way down the edges of the principal metal. Thus, when theadditional gold cover metal is plated unto the surface of the lead, theadditional cover metal does not unite with the bottom cover layer. Afterremoval of the resist, the component is subjected to the same etchingconditions as used in Example 2. As seen in FIG. 23, the copperprincipal metal is severely attacked during the etching procedure usedto remove the conductive layer.

What is claimed is:
 1. A method of making metallic features on a support comprising the steps of: (a) providing a resist on said support so as to leave openings in said resist at locations where metallic features are to be formed; (b) depositing a principal metal on said support in said openings to form a principal metal deposit with a top surface remote from the support and edge surfaces extending downwardly from the top surf ace toward the support; (c) treating said resist so as to remove resist immediately adjacent said principal metal and form gaps between said edge surfaces and said resist and thereby expose the edge surfaces of the principal metal deposit while leaving at least some of the resist on regions of the support remote from the principal metal deposit; and (d) depositing a cover metal onto the exposed edge surfaces of the principal metal deposit while said resist remains on said regions of the support remote from the principal metal deposit.
 2. A method as claimed in claim 1 further comprising the step of removing said resist after said step of depositing said cover metal.
 3. A method as claimed in claim 2 wherein said support includes a conductive layer, said step of depositing said principal metal being performed so as to deposit said principal metal over said conductive layer, the method further comprising the step of etching said conductive layer after said step of removing said resist, said cover metal protecting said edge surfaces during said etching step.
 4. A method as claimed in claim 3 wherein said step of depositing said principal metal includes the step of electroplating said principal metal.
 5. A method as claimed in claim 3 wherein said principal metal is selected from the group consisting of copper and copper-based alloys.
 6. A method as claimed in claim 5 wherein said cover metal is selected from the group consisting of gold, platinum, nickel, tin, osmium, rhodium, chromium and alloys and combinations thereof.
 7. A method as claimed in claim 1 wherein said openings include elongated slots and said metallic features include elongated leads.
 8. A method as claimed in claim 1 wherein said step of treating said resist includes the step of plasma etching said resist.
 9. A method as claimed in claim 8 wherein said plasma etching step includes the step of applying an electrical potential to said plasma in the vicinity of said principal metal deposit.
 10. A method as claimed in claim 9 wherein said electrical potential includes an alternating potential.
 11. A method as claimed in claim 9 wherein said step of applying an electrical potential to said plasma includes the step of applying an electrical potential to said principal metal deposit.
 12. A method as claimed in claim 8 wherein said step of plasma etching said resist includes the step of selectively heating said principal metal deposit.
 13. A method as claimed in claim 1 wherein said step of treating said resist includes the step of exposing said resist to a liquid stripping solution while applying an electrical potential to said principal metal deposit.
 14. A method as claimed in claim 1 further comprising the step of applying a bottom layer of a cover metal in said openings prior to applying said principal metal, whereby said bottom layer will provide cover metal on a bottom surface of said principal metal deposit facing toward said support.
 15. A method as claimed in claim 14 wherein said step of depositing said cover metal on said edge surfaces is performed so that the cover metal on said bottom surface is continuous with the cover metal on said edge surfaces.
 16. A method as claimed in claim 15 wherein said step of depositing cover metal is performed so as to provide a layer of cover metal on the top surface of the principal metal deposit continuous with the cover metal on the edge surfaces, wherein the cover metal extends entirely around the principal metal.
 17. A method as claimed in claim 1 wherein said step of depositing cover metal is performed so as to provide a layer of cover metal on the top surface of the principal metal deposit continuous with the cover metal on the edge surfaces. 